The present invention relates generally to an implantable medical device capable of communicating with an external device. More particularly, the invention relates to a programmable implantable device that transmits and receives data from an external device via pulse position modulation, and which includes a timing generator to measure the proper delay period for responding to a trigger signal received from an external device. Still more particularly, the present invention relates to a phase measurement technique implemented in a programmable implantable device for synchronizing an asynchronous input signal from an external device to increase the resolution and throughput of the data transfer.
Disruption of natural pacemaking capabilities in the heart as a result of aging or disease is commonly treated by the insertion into a patient of an artificial cardiac pacing device, commonly referred to as a pacemaker. A pacemaker provides rhythmic electrical discharges that are applied to the heart at a desired rate from the implanted artificial pacemaker. In its simplest form, the pacemaker consists of a pulse generator powered by a self-contained battery pack, and a lead including at least one stimulating electrode(s) for delivery of electrical impulses to excitable myocardial tissue in the appropriate chamber(s) in the right side of the patient's heart. However, in some instances epicardial electrodes are implanted by surgically splitting the patient's chest or other well known techniques, and suturing or screwing them in to the epicardium. Typically, the pulse generator is surgically implanted in a subcutaneous pouch in the patient's chest. In operation, the electrical stimuli are delivered to the excitable cardiac tissue via an electrical circuit that includes the stimulating and reference electrodes, and the body tissue and fluids.
Pacemakers range from the simple fixed rate device that provides pacing with no sensing function, to highly complex models implemented to provide fully automatic dual chamber pacing and sensing functions. The latter type of pacemaker is the latest in a progression toward physiologic pacing, that is, the mode of artificial pacing that restores cardiac function as much as possible toward natural pacing.
Historically, pacemakers have been employed primarily for the treatment of bradyarrhythmias, but over the past several years cardiac pacing has found significantly increasing usage in the management of tachyarrhythmias. Anti-tachyrhythmia pacemakers take advantage of an inhibitory mechanism that acts on the secondary natural pacemakers to prevent their spontaneous automaticity, sometimes termed "postdrive inhibition" or "overdrive inhibition." In essence, the heart may be driven (stimulated) with faster than normal pacing rate to suppress ectopic activity in the form of premature atrial or ventricular contractions (extrasystoles) that might otherwise initiate supraventricular or ventricular tachycardia, flutter (typically, a tachyrhythmia exceeding 200 bpm), or fibrillation; or to terminate an existing tachyrhythmia.
The pulses delivered to the heart for pacing therapy need only be of sufficient magnitude to stimulate the excitable myocardial tissue in the immediate vicinity of the pacing electrode. In contrast, another technique for terminating tachycardias, termed cardioversion, utilizes apparatus to shock the heart with one or more current or voltage pulses of generally considerably higher energy content than is delivered in pacing pulses. Whether pacing or cardioverting therapy is employed in an effort to terminate a tachycardia, a considerable risk is present that the treatment itself may precipitate fibrillation.
Defibrillation ("DF"), the method employed to terminate fibrillation, involves applying one or more high energy "countershocks" to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections, allow reestablishment of an organized spreading of action potential from cell to cell of the myocardium, and thus restore the synchronized contraction of the mass of tissue.
Typically, and as shown in FIG. 1, an implantable medical device 12, such as a pacemaker, for example, comprises electrical circuits that are controlled by processing circuitry 26, such as a central processing unit (CPU) or microprocessor. Because of the implementation of the microprocessor in the pacemaker or other implantable device, the pacemaker can be programmed by a physician through an external device 10 to customize the operation of the device to the patient's conditions. As shown in FIG. 1, the pacemaker or other implantable device 12 includes a coil antenna 30 which is capable of communicating through electromagnetic waves to a coil antenna 24 in the external programmer/reader 20 in the external device 10. The pacemaker can be programmed after it is implanted in the patient 14 through electromagnetic signals transmitted by the external programmer. The pacemaker 12 attaches to the patient's heart 16 through electrical leads 18. The pacemaker also includes a communications interface 28 to transmit and receive signals through an antenna 30. Similarly, the external device 10 also includes a communications interface 22 connected to antenna 24.
In addition to the microprocessor, the pacemaker also may include a memory device (not shown), such as random access memory (RAM) chips, for storing signals indicative of the patient's health. The pacemaker may have the capability of monitoring physiological parameters of the patient, such as EKG signals, and may store digital signals representative of these parameters in the memory device. When prompted by the external device, the processing circuitry 26 can transmit the contents of the memory device to the external programmer/reader 20 for analysis by the physician.
Because of the large amount of data that may be transmitted between the pacemaker and the external device, a significant amount of time may be required to complete the data communications. The data transactions could be expedited by the use of higher speed electronics, but the use of such high speed circuitry requires large power supplies. A pacemaker includes a battery (not shown) for operating the circuitry, which preferably has a life of 5-10 years. Operating the pacemaker circuitry at higher frequencies results in greater power consumption, which could greatly curtail the life of the battery, requiring surgery to replace. Power consumption, therefore, is at a premium in such an implantable device.
In commonly assigned U.S. Pat. No. 5,383,912, entitled "Apparatus and Method for High Speed Data Communication Between an External Medical Device and an Implantable Medical Device," the teachings of which are incorporated by reference herein, the assignee of the present invention has developed a pacemaker, capable of high speed data communication, which uses energy from the signal transmitted by the external device to generate a pulse position modulated response.
Data is transferred in commonly assigned U.S. application Ser. No. 08/058,752 through the use of pulse position modulation. A trigger signal is transmitted by the external device 10, and the processing circuitry 26 in the pacemaker responds with an output signal that is transmitted to the external device after a certain delay period. The period of delay defines the response. This is done by allocating certain window periods for a response, so that the time period during which that response occurs determines the data to be conveyed. For example, after the trigger signal is received from the external device, the system might allocate sixteen window periods for a response to represent the transfer of four digital bits (2.sup.4 =16). The time during which the response is sent indicates the digital data being transferred. Thus, if the response fell in the ninth available window period, a digital 9, or 1001, might be indicated. Subsequent data transfers occur in similar fashion until all data has been transferred. This procedure is implemented by including a digital counter or timing generator in the implantable device. The digital timing generator is initialized on the first rising clock edge following receipt of the trigger signal. The desired delay is loaded into the digital counter, so that when the count reaches zero, an output signal is generated to the external device.
One potential shortcoming with this arrangement is the asynchronism between the external trigger signal and the internal clock signal of the implantable device. As a result, the window periods for the pulse position modulated response cannot be finely resolved. Instead, the window periods must be defined sufficiently broadly to account for delays in detecting the asynchronous trigger signal. This is necessary because the trigger signal is not synchronized with the internal system clock signal which initiates the timing generator, and thus the digital timing generator may not begin counting until a full clock cycle after the trigger signal is transmitted.
The telemetering circuitry in the pacemaker sets the delay counter for the window periods immediately upon receiving the trigger signal. The delay counter, however, does not begin counting until the subsequent rising clock edge, thereby creating a potential delay between transmission of the trigger signal and the start of the digital delay counter. As a result, the timing generator potentially does not begin until almost a full clock signal after the trigger signal. A timing diagram for a digital delay counter is shown in FIG. 2. when designing a delay counter, the possibility of the trigger occurring asynchronously relative to the internal clock must be considered. The internal clock signal drives the count-down digital timing generator on a clock edge in accordance with conventional techniques. At the clock edge (which for example may be a rising clock edge) following the occurrence of the trigger signal, the digital timing generator begins counting down the preset delay value (expressed as a multiple of the clock period). The output response is then generated when the timing generator value reaches 0, as shown in FIG. 2. Because the input trigger can occur at any point within a particular clock period, the actual delay T.sub.a is given by: EQU T.sub.A =PRESET.times.T.sub.CLK T.sub.E
where PRESET is the timer preset value, T.sub.CLK is the internal clock period, and T.sub.E is the phase uncertainty between the clock signal and the input event. The value of T.sub.e, which represents the error between the actual delay achieved and the desired delay, can vary from 0 to T.sub.CLK . Depending on the specifics of the system design, this uncertainty may exceed the required error tolerance, and hence destroy the information integrity.
One way to increase the resolution of the transmission is simply to provide a faster internal clock to minimize the possible phase uncertainty between the internal clock and the trigger input. Unfortunately, this higher clock frequency results in proportionally higher power consumption by the oscillator driving the internal clock and the digital circuitry used in the implantable device. In addition, many digital systems may not be operable at a higher clock frequency due to frequency dependencies already incorporated into their designs.
It would be advantageous, therefore, to develop a system for detecting phase uncertainty between an internal clock and a trigger event to increase the resolution and bandwidth of the data transfer.